System and method for efficient wide dynamic range coil drive

ABSTRACT

Methods and systems are provided for driving one or more force coils. A system for driving force coils is provided including a PWM drive coupled to a first coil and a linear drive coupled to a second coil. The PWM drive efficiently drives the first coil to apply a first force. The linear drive drives the second coil to apply a second force that is substantially noise-free. The first force is greater than the second force.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.02_C-5080 awarded by N.A.S.A.

FIELD OF THE INVENTION

The present invention generally relates to driving force coils and, moreparticularly, relates to systems and methods for driving force coilswhile controlling the resolution, efficiency, and noise across a widerange of force application.

BACKGROUND OF THE INVENTION

A variety of platforms (e.g., aircraft, automotive vehicles, ships,spacecraft, and buildings) have payloads (e.g., mirrors, telescopes,lasers, cameras, and other types of sensing devices) attached to themthat require a vibration free environment. Active isolators have beenused to provide the vibration free environment. The isolators performfour different primary functions: connecting the payload to theplatform; isolating the payload from the vibrations of the platform;transferring forces from the platform to the payload to change thepayload orientation or point the payload; and, transferring forces fromthe platform to the payload to cancel forces generated on the payloadthat may induce vibration of the payload.

One example of an active isolator is a passive D-strut in combinationwith a multistage (e.g., two force coils) actuator. The passive D-struthas a spring and a damper in parallel with the spring along the axis ofcontrol of the strut. The spring and damper connect the payload to thebus and make the isolator “soft” (e.g., moveable) along the axis ofcontrol and “stiff” (e.g., substantially rigid) in all other axes.Absent a force application (e.g., via the force coil), the springsupports the payload in a neutral position. The damper prevents thepayload from being driven into oscillation at the resonance frequency ofthe spring. A typical force coil is an electrical coil that whenenergized (e.g., via a force coil drive) applies equal and oppositeforces to the payload and the platform along the axis of control. Theamount of force the force coil applies is proportional to the magnitudeof the current running through the force coil, and the direction of theforce application is generally dependent on the polarity of thiscurrent.

When using the isolator to prevent the transfer of vibration movementsof the platform to the payload, the force coil is driven at a currentlevel that creates the forces for compressing and extending thespring/damper such that the payload sees no change in the force formaintaining the payload in the neutral position. These applied forcesare typically in a low force range (e.g., peak forces less than about 1pound-force (lbf)) over a frequency range of DC to 100 Hz and have aresolution of 0.0005 lbf. These forces are desirably noise/ripple free.

When using the isolator to cancel forces applied to the payload byequipment mounted on the payload, the force coil applies an equal andopposite force to transfer the force to the platform while canceling theforce on the payload thereby maintaining the payload in the neutralposition. The canceling force is typically less than about 0.1 lbf overa frequency range of about 100 Hz to about 300 Hz and having aresolution of about 0.0005 lbf. This canceling force is also desirablynoise/ripple free.

When using the isolator to transfer forces to re-orientate the payload,the forces are generally known and can be directly commanded. Theseforces are substantially greater in magnitude (e.g., up to about 100lbf) than the previous mentioned applied forces but are associated withsubstantially lower frequencies (e.g., less than about 0.01 Hz). Becausethese forces are used to re-orientate the payload, these forces are notrequired to be noise/ripple free but should be efficiently generated.However, transitioning from a high-force efficient operation (e.g., forpayload re-orientation) to a low force noise free operation (e.g., forcanceling forces applied to the payload by equipment mounted on thepayload) should be smooth to prevent applying undesirable forces thatcould disturb the payload and induce vibrations.

One conventional technique for applying some of these forces includesthe use of a single force coil driven by a single drive, such as alinear drive or a pulse width modulated (PWM) drive. The single drivecan be a single-ended or a dual-ended drive (e.g. an H-bridge drive).The single-ended drive is generally simpler to incorporate a currentsense, such as by installing a precision resistor between the force coiland a ground. Additionally, the single-ended drive may be operatedaccurately by solely supplying a drive current to the force coil andreferencing the force coil to ground to minimize common-mode offset.Most conventional drive systems are powered from a single source.Generally, the single-ended drive requires two power sources, a separatepower source for each polarity of the single-ended drive, and the secondpower source would be created to sink and source power for thesingle-ended drive. The H-bridge drive may be powered from a singlepower source referenced to ground.

Driving with a linear drive is generally considered noise free due tothe constant application of the desired voltage drop to obtain thedesired output current/force. However, the linear drive is generallyinefficient because, with the continuous application of the requiredvoltage, the linear drive drops the difference between the supplyvoltage and the desired voltage. With the average voltage drop acrossthe force coil being zero, the total power dissipation is the currentmultiplied by the supply voltage. The linear drive losses are thus thetotal power losses less the coil losses.

The PWM Drive applies two different voltages to the force coil over ashort time period, and the percentage of time in each voltage state iscontrolled such that the average voltage applied over this time periodsubstantially equals the desired average voltage over this time period.The PWM drive is generally the most efficient because the output stageof the PWM drive is being switched between two states when the outputstage is in saturation. This minimizes the power dissipated in the PWMdrive because the power dissipation is the force coil current multipliedby the saturation voltage. In an H-bridge PWM drive, the current flowsthrough two separate drives of the H-bridge PWM drive at all times suchthat the power dissipation is twice the saturation voltage multiplied bythe force coil current.

A digital control system has been used to control the force coil drivewith force sensors in active struts, accelerometers on the payload, andan inertial reference sensor system on the payload as disturbancedetectors to provide feedback for determining the force coil drivecurrents. External commands may be provided to the digital controlsystem for repositioning. In this application, the digital controlsystem outputs current commands to the force coil drive via 16-bitdigital-to-analog (D-A) converters. Current telemetry feedback is alsoprovided via 16-bit analog-to-digital (A-D) converters. More commonly,12-bit D-A converters and A-D converters are used for such applications.To control the force coils to adequately output forces for the foregoingoperations, the force coils should operate over a range of about −100lbf to about +100 lbf and with a resolution of about 0.0005 lbf.Dividing the full range (e.g., 200 lbf) by the resolution (e.g., 0.0005lbf) produces 400,000 increments or just under 23-bits of resolution.The 16-bit D-A of the digital control system has 32,768 bits ofresolution.

Accordingly, it is desirable to provide a method and system for drivingone or more force coils that may be used with an isolator. Moreparticularly, it is desirable to provide a method and system forefficiently driving one or more force coils over a wide range withhigher resolution than presently obtainable. Furthermore, otherdesirable features and characteristics of the present invention willbecome apparent from the subsequent detailed description of theinvention and the appended claims, taken in conjunction with theaccompanying drawings and this background of the invention.

BRIEF SUMMARY OF THE INVENTION

Systems and methods are provided for efficiently driving one or moreforce coils to produce output forces over a wide range with higherresolution than obtainable with conventional force coil drive systems.In an exemplary embodiment, a system for driving force coils is providedcomprising a pulse width modulation (PWM) drive having an output forcoupling to a first coil of the force coils, and a linear drive havingan output for coupling to a second coil of the force coils. The PWMdrive is configured to drive the first coil to apply a first force, andthe linear drive is configured to drive the second coil to apply asecond force. The first force is greater than the second force, and thesecond force has low noise. More particularly, the PWM drive produces afirst current signal that drives the first coil to efficiently producehigh output forces, and the linear drive produces a second currentsignal that drives the second coil to produce low output forces with apredetermined resolution.

In another exemplary embodiment, a system for driving force coils isprovided comprising first and second force coils, a PWM drive having anoutput coupled to the first force coil, and a linear drive having anoutput coupled to the second force coil. The PWM drive is configured todrive the first force coil to apply a first force, and the linear driveis configured to drive the second coil to apply a second force. Thefirst force is greater than the second force, and the second force haslow noise.

In another exemplary embodiment, a method for manufacturing a force coildrive system having a PWM drive and a linear drive is providedcomprising defining operation parameters of the force coil drive system,determining modes of operation based on one or more of the operationparameters, selecting a force coil configuration based on one or more ofthe operation parameters, determining a drive type for each of the PWMdrive and the linear drive, selecting a PWM scheme when an H-bridgedrive is selected, and selecting a force coil design.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a block diagram of a force coil drive system in accordancewith an exemplary embodiment of the present invention;

FIG. 2 is a block diagram of a force coil drive system having anH-bridge PWM Drive and a single-ended linear drive in accordance withanother exemplary embodiment of the present invention;

FIG. 3 is a block diagram of a force coil drive system having anH-bridge PWM Drive and an H-bridge linear drive in accordance withanother exemplary embodiment of the present invention;

FIG. 4 is a block diagram of a force coil drive system having parallelforce coils in accordance with an exemplary embodiment of the presentinvention;

FIG. 5 is a block diagram of a force coil drive system having parallelforce coils in accordance with another exemplary embodiment of thepresent invention;

FIG. 6 is a block diagram of a force coil drive system in accordancewith an exemplary embodiment of the present invention;

FIG. 7 is a flow diagram of a method for manufacturing a force coildrive system in accordance with an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

The present invention is a system and method for driving one or moreforce coils. More particularly, the present invention is a system andmethod for driving one or more force coils to produce output forces overa wide range (e.g., from about +100 lbf to about −100 lbf) with a higherresolution (e.g., about 0.0005 lbf) than currently obtainable from asingle coil driven by a single driver. In one exemplary embodiment, thesystem and method for driving one or more force coils operatesefficiently and has minimal noise/ripple over a lower force range (e.g.,from about +1.0 lbf to about −1.0 lbf).

Generally, the system comprises a pulse width modulated (PWM) drive fordriving at least one of the force coils to efficiently produce highforces (e.g., from about +100 lbf to about −100 lbf with a resolution ofabout 0.006 lbf), and a linear drive for driving at least one of theforce coils to produce low level, high resolution, noise/ripple freeforces (e.g., from about +2.0 lbf to about −2.0 lbf with a resolution ofabout 0.0001 lbf). With the present invention, the PWM drive may beoperated to drive the force coil(s) for maximum efficiency at high forcelevels. For low level, substantially noiseless force applications, thePWM drive can be deactivated while the linear drive provides such forceswith minimum noise. In some configurations, the drives can be operatedsimultaneously with the linear drive canceling ripple torques producedby the operation of the PWM drive.

The combination of the PWM drive and the linear drive has threeoperational modes. During a first mode (Mode-1), the PWM drive isactivated for maximum driving efficiency of the system. When operatingin Mode-1 with the linear drive disabled or deactivated, active driveoutput stage devices of the PWM drive are “off” and fly-back diodesassociated with the PWM drive carry the load current until the loadcurrent decays to zero. In an alternative embodiment, the linear driveremains active for a duration to force the current command to zero,thereby minimizing transients when the linear drive is reactivated.

During a second mode (Mode-2), both drives are activated to drive withminimum noise/ripple when the force levels are greater than thecapability of the linear drive. When operating in Mode-2 anddeactivating the PWM drive, active drive output stage devices of the PWMdrive are “off” and the fly-back diodes associated with the PWM drivecarry the load current until the load current decays to zero. In analternative embodiment with the PWM drive configured as an H-bridge PWMdrive, the H-bridge PWM drive may be controlled to operate in a statesuch that the force coil is effectively shorted causing the force coilcurrent to slowly decay to zero. This alternative embodiment is usefulin the event the back-emf voltage of the force coil(s), during Mode-2,is insufficient to cause current to flow after decaying to zero. Duringa third mode (Mode-3), the linear drive is activated and the PWM driveis deactivated for a lowest noise/ripple operation.

Each of the linear drive and PWM drive may have a variety ofconfigurations including, but not necessarily limited to, single-endedand H-bridge. During operation, a single-ended PWM drive has twooperating states: a positive supply across the force coil; and, anegative supply across the force coil. The single-ended PWM drive has asingle PWM scheme that switches between the two supply voltages toobtain the desired average voltage. An H-bridge PWM drive has fouroperating states: the supply voltage is coupled to the positive end ofthe force coil and the negative end is coupled to ground in a firststate; both ends of the force coil are coupled to ground in a secondstate; the supply voltage is coupled to the negative end for the forcecoil and the positive end is coupled to ground in a third state; and,both ends of the force coil are coupled to the supply voltage in afourth state. Both the second state and the fourth state applyapproximately zero volts across the force coil. Several potentialmodulation schemes are available with four different states. In a firstmodulation scheme, a two-state scheme, the PWM drive switches betweenthe first state and the third state. In a second modulation scheme, athree-state scheme, the PWM drive switches between the first state andthe second state or switches between the second state and the thirdstate. The PWM drive 12 can operate in either the two-state scheme orthe three-state scheme and operate in any of the three operating modes(e.g., Mode-1, Mode-2, or Mode-3).

Referring now to the drawings, FIG. 1 is a block diagram of a force coildrive system 10 in accordance with an exemplary embodiment of thepresent invention. The drive system 10 comprises an efficient PWM drive12, a coarse force coil 14 coupled to the PWM drive 12, a linear drive16, and a fine force coil 18 coupled to the linear drive 16. In thisexemplary embodiment, the coarse force coil 14 is electrically andmagnetically isolated from the fine force coil 18. The PWM drive 12efficiently drives the coarse force coil 14 to produce relatively highforces (e.g., about ±100 lbf), while the linear drive 16 drives the fineforce coil 18 to produce relatively noise/ripple-free low forces (e.g.,about ±1 lbf). The coarse force coil 14 is configured for producing therelatively high forces, and the fine force coil 18 is configured forproducing the relatively noise/ripple-free low forces. The force coildrive system 10 is highly versatile because each force coil 14, 18 canbe tuned for a particular performance. Additionally, each of the drives12, 16 can be a different drive type (e.g., one single-ended and theother an H-bridge, both single-ended, or both H-bridges), and the drives12, 16 can be powered from different supplies.

In one exemplary embodiment, with both drives 12, 16 configured asH-bridge drives, the drive system 10 can be operated between Mode-3(e.g., with PWM drive 12 deactivated for minimum noise/ripple) andMode-2 (e.g., with both drives 12, 16 activated for high forceoperation), and use either method for disabling the PWM drive 12 or thelinear drive 16 (e.g., all active devices of the H-bridge drive being“off” or the active devices of the lower output stages of the H-bridgedrive being “on”). For example, the active devices of the lower outputstages of the PWM drive 12 are “on” when the PWM drive 12 is deactivatedto produce a zero current condition during three-state operation of thePWM drive 12, and the current gradually decays to zero. When the activedevices of all output stages of PWM drive 12 are “off,” the current morerapidly decreases to zero.

FIG. 2 is a block diagram of a force coil drive system 40 having anH-bridge PWM drive and a single-ended linear drive 22 in accordance withanother exemplary embodiment of the present invention. The force coildrive system 40 comprises an H-bridge PWM drive 20 powered by anexternal power supply and ground, the coarse force coil 14 coupled tothe H-bridge PWM drive 20, a single-ended linear drive 22, and the fineforce coil 18. The linear drive 22 is identified as single-ended withone end of the fine force coil 18 coupled to ground through a currentsense resistor 28 with the linear drive 22 powered by two oppositepolarity supplies (e.g., a secondary +DC supply and a secondary −DCsupply). Force coil drive system 40 additionally comprises current senseresistors 24, 26 each having one terminal coupled to the H-bridge PWMdrive 20 and the other terminal grounded.

The H-bridge PWM drive 20 can couple each end of the coarse force coil14 to either the external DC supply or ground. Additionally, theH-bridge PWM drive 20 receives control logic driving the H-bridge PWMdrive 20 that assures that current is sourced or returned at one end ofthe coarse force coil 14, at the other end of the coarse force coil 14,or at both ends at all times so that the load current is sensed bymeasuring the voltage drop across one of the current sense resistors 24,26. The measured current can be used as a feedback to control theduty-cyle of the H-bridge PWM drive 20. The single-ended linear drive 22operates between a positive and negative supply (e.g., the secondary +DCsupply and secondary −DC supply) with respect to ground. With thecurrent sense resistor 28 coupled to ground, common mode offset, thatmay be introduced into the feedback loop of the single-ended lineardrive, is minimized.

FIG. 3 is a block diagram of a force coil drive system 42 having anH-bridge PWM drive 20 and an H-bridge linear drive 34 in accordance withanother exemplary embodiment of the present invention. The linear drive34 is identified as an H-bridge type by being powered between theexternal DC supply and ground. The coarse force coil 14 continues to bedriven by the H-bridge PWM drive 20, and inductors 30 and 32 are coupledbetween the coarse force coil 14 and the H-bridge PWM drive 34. Currentsense resistors 36 and 38 are also coupled in series with the fine forcecoil 18. The inductor 30 is coupled between the H-bridge PWM drive 20and one end of the coarse force coil 14, and the inductor 32 is coupledbetween the H-bridge PWM drive 20 and the other end of the coarse forcecoil 14. The inductors 30 and 32 add additional series inductance withthe inductance of the coarse force coil 14. In this exemplaryembodiment, the inductors 30 and 32 isolate the coarse force coil 14from the sharp voltage edges of the H-bridge PWM drive 34 which mayotherwise result in conducted or radiated noise without such inductors30 and 32. Additional resistors (not shown) can be added paralleling theinductors 30 and 32 to dampen resonances that might be created by theinductors 30 and 32 and potential stray capacitance across the forcecoils 14, 18.

FIG. 4 is a block diagram of a force coil drive system 44 havingparallel force coils 14, 18 in accordance with an exemplary embodimentof the present invention. The drive system 44 comprises the efficientPWM drive 12, the coarse force coil 14, the linear drive 16, and thefine force coil 18. In this exemplary embodiment, the coarse force coil14 and fine force coil 18 are identical non-magnetically coupled coilsthat are coupled in parallel. Both the PWM drive 12 and the linear drive16 drive the parallel coupled force coils 14 and 18. For example, afirst end of each of the force coils 14, 18 is coupled to the PWM drive12 and the linear drive 16, and a second end of each of the force coils14, 18 is coupled to the PWM drive 12 and the linear drive 16. Bothforce coils 14 and 18 are used to output a maximum force, and the drivesystem 44 can be operated to provide a smooth transition as the lineardrive 16 operates to carry the PWM drive 12 current thereby increasingefficiency. The force coils 14, 18 are configured to meet both coiloperating characteristics and can increase the PWM drive 12 currentlevel.

The drive system 44 operates in Mode-1 (e.g., with only the PWM drive 12“on”) or in Mode-3 (e.g., with only the linear drive 16 “on”), and thedrives 12 and 16 are both either H-bridge drives or single-ended drives.The PWM drive 12 can operate in either two-state or three-state. In theevent the linear drive 16 is a current source drive, both drives 12 and16 can be simultaneously operated. The linear drive 16 is preferablyactivated before the PWM drive 12 is turned “off” when switching fromMode-1 to Mode-3. When the PWM drive 12 is deactivated, all outputstages of the PWM drive 12 are turned “off” for operation of the lineardrive 16. When the PWM drive 12 is re-activated, the PWM drive 12 ispreferably activated before the linear drive 16 is deactivated forminimum transition disturbance. When deactivated, the linear drive 16has all output stages “off” or produces a zero current command.

FIG. 5 is a block diagram of a force coil drive system 46 havingparallel force coils 14, 18 in accordance with another exemplaryembodiment of the present invention. In this exemplary embodiment, thecoarse force oil 14 and fine force coil 18 are coupled in parallel. ThePWM Drive 12 is isolated from the parallel coupled force coils 14, 18 byseries inductors 30 and 32, and the linear drive 16 is coupled directlyacross the parallel coupled force coils 14, 18. For example, each of theinductors 30, 32 has one end coupled to the PWM drive 12 and another endcoupled to a different end of the parallel coupled force coils 14, 18.In this exemplary embodiment, drive system 46 can operate in Mode-1,Mode-2, or Mode-3 with sufficient inductance provided by inductors 30,32, and the drives 12 and 16 are both either H-bridge drives orsingle-ended drives. Both drives drive the parallel coupled force coils14, 18 and are thus capable of outputting high forces while having thebandwidth for low force applications.

FIG. 6 is a block diagram of a force coil drive system 50 in accordancewith another exemplary embodiment of the present invention. In thisexemplary embodiment, two force coils 60 and 62 are magnetically coupledto one another, thereby maximizing the total inductance of the forcecoils 60, 62, and replace the coarse force coil 14 shown in FIG. 5. Thefine force coil 18 is magnetically isolated from the coarse force coils60, 62. Each of the force coils 60, 62 is coupled in series with thefine force coil 18 such that each of the force coils 60, 62 has one endcoupled to a different end of the fine force coil 18. The drive system50 also comprises inductors 30 and 32 that are each coupled to adifferent one of the force coils 60, 62. For example, a first end of theforce coil 60 is coupled to the inductor 30, a second end of the forcecoil 60 is coupled to a first end of the fine force coil 18, a first endof the force coil 62 is coupled to the inductor 32, and a second end ofthe force coil 62 is coupled to a second end of the fine force coil 18.The inductors 30, 32 may be omitted from drive system 50.

The PWM drive 12 is coupled to the free ends of the inductors 30, 32,and the linear drive 16 is coupled directly across the fine force coil18. The coarse force coils 60 and 62 isolate the PWM drive 12 from thelinear drive 16. In this exemplary embodiment, the drive system 50 isefficient because all of the coil volume is used to minimize the powerloss of the PWM drive 12. Additionally, this exemplary embodiment allowsfor tuning (e.g., selection of the force coil weight and size) the fineforce coil 18 separately from the coarse force coils 60, 62 forbandwidth requirements. The drive system 50 can operate in Mode-1,Mode-2, or Mode-3 with sufficient inductance provided by the coarseforce coils 60, 62 and the inductors 30, 32. The drives 12, 16 are bothH-bridge drives or single-ended drives. The PWM drive 12 operates intwo-state in the event the linear drive 16 is simultaneously operated(e.g., during Mode-3). When the PWM drive 12 is deactivated, all outputstages of the PMW drive 12 are “off” to allow the linear drive 16 tooperate. When deactivated, the linear drive 16 has all output stages“off” or produces a zero current command. The coil tuning is preferablyaccomplished by adjusting the ratio of physical dimensions of the coarseforce coils 60, 62 and the fine force coil 18 rather than adjusting thewires sizes of the force coils 60, 62, 18.

FIG. 7 is a flow diagram of a method 200 for manufacturing a force coildrive system in accordance with an exemplary embodiment of the presentinvention. The drive system comprises a PWM drive and a linear drive.Operation parameters of the drive system are determined as indicated atstep 105. For example, the high force range and corresponding bandwidthrequirements are initially determined for a desired drive systemefficiency. Additionally, the minimal noise/ripple range of operationfor the drive system and the corresponding bandwidth and resolutionrequirements are determined. Modes of operation of the drive system areestablished as indicated at step 110. In an exemplary embodiment, themodes of operation are selected from one or more combinations of Mode-1,Mode-2, and Mode-3 (e.g., a first combination of Mode-1 and Mode-2, asecond combination of Mode-2 and Mode-3, a third combination of Mode-1and Mode-3, and a fourth combination of Mode-1, Mode-2, and Mode-3). Theoperating relationship between the high force range and noise/ripplerange may influence the selection of the modes of operation. In someapplications, the drive system can separate operation to provide theforces for each of the ranges. In other applications, the drive systemis limited to a particular mode of operation. For example, to cancel PWMripple using the linear drive, the drive system operates in Mode-3. Aforce coil configuration is selected as indicated at step 115. The forcecoil configuration may be selected from a parallel coil configuration, aseries coil configuration, and an isolated coil configuration. Theselected modes of operation may also influence this selection. Forexample, the parallel coil configurations may be selected for higherefficiency operation, and the series or isolated coil configurations maybe selected to accommodate bandwidth requirements. The type of drive forthe PWM drive and the linear drive is selected as indicated at step 120.For example, the PWM drive and the linear drive may be selected to beboth H-bridge drives, both single-ended drives, or one H-bridge driveand one single-ended drive. A determination is made as to the selectionof an H-bridge drive as indicated at step 125. In the event an H-bridgedrive is selected, a PWM scheme is selected as indicated at step 130.For example, a two-state PWM scheme or a three-state PWM scheme isselected for the drive system. Additionally, a PWM disabled statecondition and a linear drive disabled state condition are determined.For example, the PWM disabled state condition is determined as eitherhaving all output stages of the PWM drive being “off” or having thelower output stages of the PWM drive constantly “on” (e.g., duringtwo-state PWM operation). Additionally, the linear drive disabled statecondition is determined as either having all output stages of the lineardrive being “off” or having the linear drive transmit a zero currentcommand. The force coil design is determined as indicated at step 135.For example, the weight and size of each of the force coils (e.g.,coarse force coil and fine force coil) is optimized for a particularcoil configuration.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

1. A system for driving force coils, the system comprising: a pulsewidth modulation (PWM) drive having an output coupled to a first coil ofthe force coils, said PWM drive configured to drive the first coil toapply a first force; and a linear drive having an output coupled to asecond coil of the force coils, said linear drive configured to drivethe second coil to apply a second force, said first force greater thansaid second force, said second force having low noise.
 2. A systemaccording to claim 1, wherein said PWM drive and said linear drive aretogether configured to operate in a mode selected from the groupconsisting of: a first mode comprising said PWM drive being activatedand said linear drive being deactivated; a second mode comprising saidPWM drive and said linear drive both being activated; and a third modecomprising said linear drive being activated and said PWM drive beingdeactivated.
 3. A system according to claim 1, wherein whentransitioning from said first mode to said third mode, said linear driveis configured to be activated prior to said PWM drive being deactivated.4. A system according to claim 1, wherein the first coil has first andsecond ends; wherein each of said PWM drive and said linear drivecomprises an H-bridge drive; and wherein said PWM drive furthercomprises: a first operating state, said PWM drive configured to couplethe first end of the first coil to a supply potential and coupling thesecond end of the first coil to a ground during said first operatingstate; a second operating state, said PWM drive configured to couple thefirst and second ends of the first coil to said ground during saidsecond operating state; a third operating state, said PWM driveconfigured to couple the first end of the first coil coupled to saidground and couple the second end of the first coil to said supplypotential during said first operating state; and wherein said PWM driveis further configured to operate in one of a two-state PWM scheme and athree-state PWM scheme, said PWM drive configured to switch between saidfirst operating state and said third operating state during saidtwo-state PWM scheme, said PWM drive further configured to switchbetween said first operating state and said second operating state andswitch between said second operating state and said third operatingstate during said three-state PWM scheme.
 5. A system for driving forcecoils, the system comprising: a first force coil; a pulse widthmodulation (PWM) drive having an output coupled to said first forcecoil, said PWM drive configured to drive said first force coil to applya first force; a second force coil; and a linear drive having an outputcoupled to said second force coil, said linear drive configured to drivesaid second coil to apply a second force, said first force greater thansaid second force, said second force having low noise.
 6. A systemaccording to claim 5, wherein said PWM drive is further configured todrive said first force coil to apply from about −100 lbf to about +100lbf; and wherein said linear drive is further configured to drive saidsecond force coil to apply from about −1 lbf to about +1 lbf.
 7. Asystem according to claim 5, wherein said PWM drive is furtherconfigured to drive said first force coil with a resolution of about0.006 lbf, and wherein said linear drive is further configured to drivesaid second force coil with a resolution of about 0.0001 lbf.
 8. Asystem according to claim 5, wherein said PWM drive comprises anH-bridge PWM drive having a first input configured to couple to aprimary direct current (DC) supply and having a second input configuredto couple to a reference potential; wherein said linear drive comprisesa single-ended linear drive having first and second inputs, said firstinput of said single-ended linear drive configured to couple to a +secondary DC supply, said second input of said single-ended linear driveconfigured to couple to a − secondary DC supply; wherein said secondforce coil has first and second ends; and wherein the system furthercomprises a resistor having a first end coupled to said first end ofsaid second force coil and having a second end coupled to said referencepotential, said second end of said second force coil coupled to saidoutput of said linear drive.
 9. A system according to claim 5, whereinsaid PWM drive comprises an H-bridge PWM drive having a first inputconfigured to couple to a DC supply and having a second input configuredto couple to a reference potential; and wherein said linear drivecomprises an H-bridge linear drive having an input configured to coupleto said DC supply and having a second input configured to couple to saidreference potential.
 10. A system according to claim 9, wherein each ofsaid first and second force coils has first and second ends; and whereinthe system further comprises: a first inductor having a first endcoupled to said first end of said first force coil and having a secondend coupled to said H-bridge PWM drive; a second inductor having a firstend coupled to said second end of said first force coil and having asecond end coupled to said H-bridge PWM drive; a first resistor having afirst end coupled to said first end of said second force coil and havinga second end coupled to said H-bridge linear drive; and a secondresistor having a first end coupled to said second end of said secondforce coil and having a second end coupled to said H-bridge lineardrive.
 11. A system according to claim 5, wherein said PWM drive andsaid linear drive are selected from the group consisting of bothH-bridge drives and both single-ended drives; and wherein said first andsecond force coils are non-magnetically coupled in parallel.
 12. Asystem according to claim 11, wherein said first and second force coilseach have first and second ends; and wherein the system furthercomprises: a first inductor having a first end coupled to said PWM driveand having a second end coupled to said first end of said first forcecoil; and a second inductor having a first end coupled to said PWM driveand having a second end coupled to said second end of said first forcecoil; and wherein said PWM drive is magnetically isolated from saidfirst and second force coils via said first and second inductors.
 13. Asystem according to claim 5, wherein said first force coil comprisesthird and fourth force coils, said third force coil magnetically coupledto said fourth force coil, said second force coil magnetically isolatedfrom said third and fourth force coils, each of said second, third, andfourth force coils having first and second ends, said first end of saidthird force coil coupled to said PWM drive, said second end of saidthird force coil coupled to said first end of said second force coil,said first end of said fourth force coil coupled to said PWM drive, saidsecond end of said fourth force coil coupled to said second end of saidsecond force coil; and wherein said linear drive is coupled across saidsecond force coil.
 14. A system according to claim 5, wherein said firstforce coil comprises third and fourth force coils, said third force coilmagnetically coupled to said fourth force coil, said second force coilmagnetically isolated from said third and fourth force coils, each ofsaid second, third, and fourth force coils having first and second ends;and wherein the system further comprises: a first inductor having afirst end coupled to said PWM drive and having a second end coupled tosaid first end of said third coil, said second end of said third coilcoupled to said first end of said second coil; and a second inductorhaving a first end coupled to said PWM drive and having a second endcoupled to said second end of said fourth coil, said second end of saidfourth coil coupled to said second end of said second coil.
 15. A methodfor manufacturing a force coil drive system having a pulse widthmodulation (PWM) drive and a linear drive, the method comprising thesteps of: defining operation parameters of the force coil drive system;determining modes of operation based on one or more of the operationparameters; selecting a force coil configuration based on one or more ofthe operation parameters; determining a drive type for each of the PWMdrive and the linear drive; selecting a PWM scheme when an H-bridgedrive is selected; and selecting a force coil design.
 16. A methodaccording to claim 15, wherein said step of defining operationparameters comprises: defining a high force range and a first bandwidthbased on the high force range; and defining a minimal noise range, asecond bandwidth based on the minimal noise range, and a resolutionbased on the minimal noise range.
 17. A method according to claim 15,wherein said step of determining modes of operation comprises combiningat least two modes from the group comprising: activating only the PWMdrive; activating the PWM drive and the linear drive together; andactivating only the linear drive.
 18. A method according to claim 15,wherein said step of selecting a force coil configuration comprisesselecting the force coil configuration from a parallel coilconfiguration, a series coil configuration, and an isolated coilconfiguration.
 19. A method according to claim 15, wherein said step ofselecting a PWM scheme comprises selecting the PWM scheme from atwo-state PWM and a three-state PWM.
 20. A method according to claim 15further comprising determining a PWM disable state condition and alinear drive disable state condition based on the modes of operation.